Computerized color graphics systems and electronic printing systems are known in the art. Typically, they enable a user to produce a color image, and from that image to produce a picture which can be printed via a color reproduction system, for example, by producing color separation plates for offset printing. There has been much effort in the past to develop ways to accurately predict the appearance of such images when printed on a substrate, for example paper or film, using a number of colorants, such as inks, the prediction carried out without actually printing the images. It should be noted that an image in the context of the present invention, may include text, graphic line art, continuous tone images, and/or any visual two-dimensional pattern.
Often, printing is carried out using halftoning. Halftoning, also called screening, is the process of creating the illusion of a continuous tone ("CT," "contone") image using an output (e.g., printing) device capable only of binary output (ink deposited or not deposited at any location on a substrate). Halftoning involves the placement of binary (ink or no ink) picture elements, and the amount of ink picture elements placed per unit area ("dot percentage") determines the tone. For color printing, several images ("separations") are produced in the primary colorants (typically inks) used to print in color, and overlaid in printing. For typical four color printing, four images are produced in cyan ("C"), magenta ("M"), yellow ("Y") and black ("K"), and each of these images are halftoned. Halftoning is also called screening because historically in pre-electronic days, a physical screen was used. If a halftone pattern is regular, the individual halftone cells are called "screen dots" and they are said to be part of a "screen." Usually, digital halftoning is used together with an imagesetter, laser printer, ink jet printer, digital film recorder, or other recorder output device. Each of these devices has a fundamental recording element, herein called a pixel, which represents an element on which ink can be deposited, or not. This smallest unit on a recorder is called a "microdot," "recorder element," "recorder pixel," or the like. Its size is called the "recorder pitch," "recorder resolution," "imagesetter resolution," etc., and is expressed either as units of recorder element size, e.g, 1/1800 of an inch, or a spatial frequency, e.g., 1800 pixels per inch (ppi) or 1800 dots per inch (dpi). Some presses nowadays have pixels which can have intermediate levels between on and off. This is for instance obtained by the amount of colorant (typically ink) deposited on that pixel. For example, in a four level printer, there can be 0%, 33%, 67% or 100% of the maximum amount of ink applied on any one pixel element. It is then said for such a printer that a pixel has a 2-bit value, instead of a 1-bit value.
It is desirable to be able to calculate very accurately how a picture will look when one prints an image, including a halftoned continuous tone color image, with a certain technique on a certain substrate using a certain set of colorants (e.g., inks). If one knows before actually printing what a picture will look like after reproduction, one clearly can save a lot of time and money. For example, a method of predicting the color appearance can be used to display a simulation of the color appearance on a computer display, or to print a simulation of the color appearance on a more easily accessible and cheaper printer as a proof of what is to be printed finally in production.
It is desirable to do this for both reflection printing on an opaque substrate, and transmission imaging on a transparent substrate.